A woman who had part of her jaw cut away after being wrongly diagnosed with cancer of the mouth has received an apology from the University of Otago Dental Hospital.

The surgeon who treated her says the misdiagnosis happened after a laboratory worker at Medlab Dental, in Dunedin, dropped two samples on the floor and mixed them up.

University Pro-Vice Chancellor health sciences and faculty of medicine dean Prof Peter Crampton said the pathology laboratory was part of the University of Otago Dental Hospital.

"The patient was contacted very soon after the incident was discovered, and we offered a full apology at that time,'' Prof Crampton said.

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"We have taken this incident very seriously, and have already taken all appropriate measures to minimise the likelihood of any such incidents occurring again.''

Last month, Southern Community Laboratories in Dunedin apologised to an Otago woman who had a mastectomy by mistake earlier this year, after her breast biopsy specimen was swapped with another.

The switch resulted in a false clearance for the other Otago woman.

In the latest case, Nelson oral surgeon Dr Iain Wilson said his 63-year-old patient suffered facial swelling and sinus infections after having a tooth implant last year.

Dr Wilson took an oral biopsy sample, which was sent to Medlab Dental and tested for cancer.

The results were positive and the woman underwent a hemimaxillectomy to remove the right side of her upper jaw.

Bone and artery veins were taken from her lower leg and used to reconstruct the woman's jaw, Dr Wilson said.

"Her donor wound site got infected and she had difficulty walking,'' Dr Wilson said.

She was later told that tissue taken during the surgery showed no sign of cancer and her initial specimens had been swapped with another patient after the pathologist accidentally dropped her test pot.

"I am being asked to believe two samples were being processed simultaneously and the pots were simultaneously dropped,'' Dr Wilson said.

"I can't for the life of me understand how you can get tissue samples mixed up,'' he said.

"I am astonished and horrified by these lab mix-ups.''

He said the case was before the Health and Disability Commission and the patient had consulted a lawyer about compensation.

She was the sixth woman the Herald on Sunday had discovered to be affected by errors made in pathology laboratories since it started an investigation a month ago.

A senior pathologist said more mistakes were inevitable if pathology processes and funding structures continue to lag behind international standards.

Auckland senior surgical pathologist Tony Bierre claimed cancer pathology was in the midst of a crisis and urgent action was needed to prevent further errors which had life-changing consequences for patients.

Associate Health Minister Jo Goodhew said last week the Government had extended its inquiry into botched biopsies to cover all laboratory processes relating to biopsies.

The original inquiry followed revelations that at least three women had breasts removed after wrong cancer diagnoses.